Display devices, such as desk-top computer screens and direct view and projection television sets, include electron excited fluorescent display devices such as cathode ray tubes. Cathode ray tubes (CRTs) function as a result of a scanning electron beam from an electron gun impinging on phosphors on a relatively distant glass screen. The phosphors absorb the energy from the electron beam and subsequently emit a portion of the energy, which is typically in the visible region of the electromagnetic spectrum. This visible emission is then transmitted through the glass screen to the viewer. Other display devices, such as field emission displays for use in flat panel display screens, which include cold cathode emission devices, and vacuum fluorescent displays for use in handheld calculators, which include hot cathode emission devices, also function as a result of electrons exciting phosphors on a screen.
Phosphors are inorganic luminescent materials that typically include "activator" atoms that can modify the emitted radiation, such that the emission is in the visible region, as well as modify the emission intensity and the persistence of the image. Phosphors should preferably be capable of maintaining luminescence (e.g., fluorescence) under excitation for a relatively short period of time to provide superior image reproduction. This is a characteristic known as "persistence." Typically, persistence for display phosphors is less than about 50 milliseconds. Persistence that is too long can be undesirable in display devices, causing a smearing effect on the screen, but may be desirable in a fluorescent light tube or the like. Another characteristic of phosphors is lifetime, which refers to the degradation of the phosphor over the time of use, which may extend to months or years of normal use, depending upon the application.
In general, it is desirable to produce highly pure phosphors to increase absorption of the available excitation energy by the activator that emits the required radiation, rather than being consumed by other impurities or "killer" centers, which would result in lower luminescence and lower efficiency. Therefore, the quality of the deposited phosphor is an important parameter.
Typically, in field emission displays, cathode ray tubes, powder electroluminescent cells, and other electroluminescent articles, a phosphor is deposited on an insulating substrate, which can be coated with a transparent, conductive material such as indium tin oxide (ITO) or tin oxide (TO). In conventional cathode ray tubes (CRTs), a reflective layer (such as aluminum) is evaporated on one side of the insulating substrate in order to reflect emitted light. The reflective layer and the transparent conductive material assist in dissipating charge which builds up during use.
Commonly used methods for depositing phosphors include settling techniques, slurry methods (such as screen printing, spin coating, and spin casting), electrophoresis, or dusting methods (such as electrostatic dusting, "phototacky" methods, and high pressure dusting).
In settling methods, phosphor is suspended in a solution and allowed to settle gravitationally or centrifugally onto the substrate. In slurry methods, phosphors typically are mixed with a liquid, such as a photoresist, to form a slurry or suspension. The slurry is deposited onto the substrate, dried, and exposed to ultraviolet radiation through a photomask to produce a desired pattern. Exposed areas will be washed away during development when using a positive photoresist system, for example.
In electrophoresis, phosphor particles are deposited from a suspension under the action of an electric field. They may be deposited in a pattern onto the substrate. The suspension typically includes a nonaqueous liquid, such as an alcohol, and an electrolyte, such as a salt of yttrium, cerium, indium, aluminum, magnesium, lanthanum, or thorium. Such metal salts make it possible to electrically charge the phosphor particles. The part coated typically serves as the cathode (cataphoresis). An electrochemical reaction occurs at the cathode, believed to convert metal salts to metal hydroxides, thus assisting in phosphor adhesion.
In dusting methods, a photoresist is applied to a substrate and partially dried. Phosphor particles are then blown onto the surface, sticking to the partially dried photoresist. The layer is then completely dried and exposed to radiation through a photomask to produce a desired pattern. Unexposed areas are removed during development if a negative photoresist system is used. Electrostatic dusting is carried out in a similar manner, except that electrostatic charge is used to attract the phosphor particles onto the partially dried photoresist. In "phototacky" methods, photoresist is applied and dried to a substrate, followed by exposure to radiation through a photomask. The photoresist becomes tacky in the exposed areas and phosphor is applied to the surface, sticking only to the tacky areas.
After a phosphor screen is in use (such as for a cathode ray tube or a field emission display device), other problems can develop. For example, when a voltage is applied to a phosphor screen, electrochemical reactions can occur which result in poisoning of the cathode. In addition, reactions can occur which degrade the color and/or the intensity of phosphor emission. This can be the result if applied voltage exceeds the breakdown voltage of any of the constituents of the screen.
Thus, there is a need in the art for electroluminescent materials that have good performance in terms of the ability to conduct excess charge away from a charged phosphor screen, and an ability to reduce or prevent poisoning of the cold cathode and phosphor degradation due to unwanted electrochemical reactions.